In many bacteria, the production of transporter proteins involved in manganese uptake is under the control of manganese-activated transcriptional regulators. The streptococcal cell adhesion regulator (ScaR) from Streptococcus gordonii and manganese transport regulator (MntR) from Bacillus subtilis are specifically activated for DNA-binding by manganese. The mechanism of these proteins can be separated into two features: (1) the specific recognition of manganese, and (2) the transition from an inactive to active form upon manganese binding. The goal of this work is to define the underlying molecular mechanisms by which MntR and ScaR are able to selectively recognize and respond to manganese as an allosteric effector. X-ray crystallography, spectroscopic techniques, including fluorescence spectroscopy and EXAFS, and solution binding assays will be used to investigate the interaction of MntR and ScaR with metal ions. Site-directed mutagenesis will be used to create variants of these proteins, which will in turn be evaluated for changes in conformation and metal-binding properties. Manganese is an essential nutrient for all organisms and can be a limiting nutrient for many pathogenic bacteria, including S. gordonii, which respond to low manganese concentrations by producing proteins associated with virulence. Metalloregulatory proteins, such as MntR and ScaR, that control production of these virulence factors may be useful targets for antibiotic development. A deeper understanding of their structure and function will be important in efforts to disrupt manganese homeostasis in bacteria. Manganese is an essential nutrient for many disease-causing bacteria, and disruption of manganese uptake into the cell can have significant effects on virulence. This research is intended to provide a molecular-level understanding of how bacteria specifically recognize and respond to varying levels of manganese. That understanding may open the way to future antibiotic development. [unreadable] [unreadable] [unreadable]